The potential shortage of traditional petroleum reserves and the increasing instability of international hydrocarbon markets have prompted a search for processes to convert a range of feedstocks to low, intermediate and high boiling range hydrocarbons, including alkanes and olefins. Such alkanes and olefins can be useful in the production of fuels such as gasoline and middle distillate fuels, as speciality solvents, as chemical intermediates, as components of drilling mud oils and in the production of lubricants. Alkanes having 10 to 20 carbon atoms (C10-20 alkanes), for example, are particularly valuable as distillate-range transport fuels, such as diesel and jet fuels. Olefins can be used as precursors for a wide variety of chemical and petrochemical products, such as in the preparation of various derivative end products for the manufacture of chemicals.
The Fischer-Tropsch process can be used to convert syngas (a mixture of carbon monoxide, hydrogen and typically also carbon dioxide) into liquid hydrocarbons. Syngas can be produced through processes such as partial oxidation or steam reforming of hydrocarbons. Feedstocks for syngas production include biomass, natural gas, coal or solid organic or carbon-containing waste or refuse. One way of accessing remote natural gas is to convert it into liquid hydrocarbons (via syngas) and to transport the resulting liquid products. This “on-site” processing of the natural gas into liquid products, often termed Gas To Liquids (GTL), can avoid the need for expensive infrastructure such as long distance pipelines, or cryogenic storage and transport facilities that are needed to distribute it as liquefied natural gas (LNG). As oil reserves are depleted, and as oil prices increase, there is increasing incentive to convert such remote natural gas resources into commodity liquid fuels and chemicals.
Fischer-Tropsch synthesis can be tuned to convert syngas to a selective product distribution of olefinic hydrocarbons also containing paraffins, in varying olefin/paraffin ratios, depending on the catalyst composition, pre-treatment procedures and reaction conditions. Catalysts having various combinations of elements have been tested in the past. Fischer-Tropsch catalysts can contain Group VIII transition metals, typically cobalt, iron or ruthenium in combination with various promoters (U.S. Pat. No. 5,100,856).
The Fischer Tropsch reaction is highly exothermic, requiring rapid heat removal. Since the discovery of Fisher-Tropsch synthesis (FTS) over eighty-five years ago, only three major designs for the reactor bed found their way to commercial scale plants. Originally tubular fixed-bed reactors were utilised, but single pass conversions were generally limited to a maximum of 60% in order to control the heat of reaction. Fluidized bed and slurry reactors were subsequently developed to overcome this limitation.
U.S. Pat. No. 7,012,102 describes a Fischer-Tropsch process, which is preferably a slurry phase process, in which light saturated hydrocarbons are separated from the reaction products and fed to a dehydrogenation reactor to produce some unsaturated hydrocarbons, and recycling at least some of the unsaturated hydrocarbons to the reactor. The presence of olefins in the reactor can help increase the length of hydrocarbon chains that are produced by the reaction.
U.S. Pat. No. 6,331,573 describes an integrated process for producing liquid fuels from syngas via a two-stage Fischer-Tropsch reaction, in which the first stage uses conditions in which chain growth probabilities are low to moderate, and the product includes a relatively high proportion of C2-8 olefins and a low quantity of C30+ waxes, which product is fed to a second stage where chain growth probabilities are relatively high, and wherein light and heavier olefins compete for chain initiation. Most chains are initiated at the C2-8 olefins, and the second stage produces a larger fraction in the C5-12 range, and a low quantity of waxes.
U.S. Pat. No. 6,897,246 describes a Fischer-Tropsch hydrocarbon synthesis process, in which a C2-C9 olefin-rich stream is separated from a hydrocarbon product stream produced in the reactor to form a light olefin recycle stream, where the light olefin recycle stream is recycled to the reactor system at a point where the H2:CO molar ratio is low relative to the H2:CO ratio in the rest of the reactor system.
US 2002/0120018 relates to an integrated process for improving hydrocarbon recovery from a natural gas resource, by removing heavier hydrocarbons from natural gas, converting methane to syngas, which is then subjected to hydrocarbon synthesis, preferably Fischer-Tropsch synthesis. The produced hydrocarbons are separated into a C1-4 fraction, a fraction generally comprising C5-20 hydrocarbons, and a fraction generally comprising C20+ hydrocarbons.
US 2004/0074810 relates to the production of hydrocarbons in the kerosene/diesel boiling range from a Fischer-Tropsch process, in which (1) hydrocarbons from the Fischer-Tropsch reactor are hydrocracked/hydroisomerised, (2) separating the hydrocarbons into one or more light fractions boiling below the kerosene/diesel boiling range, one or more fractions boiling in the kerosene/diesel boiling range and a heavy fraction boiling above the kerosene/diesel boiling range, (3) subjecting the major part of the heavy fraction to hydrocracking/hydroisomerisation, (4) separating the product stream from (3) into one or more light fractions boiling below the kerosene/diesel boiling range, one or more fractions boiling in the kerosene/diesel boiling range and a heavy fraction boiling above the kerosene/diesel boiling range and (5) hydrocracking/hydroisomerising the major part of the heavy fraction from (4) in the hydrocracking/isomerising process of (1) or (3).
Challenges to optimize existing commercial reactors or to consider alternative designs for FTS processes still exist, in view of the complex nature of the synthesis process as well as the difficulty to control the thermo physical characteristics of the reaction mixture.
In typical FTS reactions carried out in 2 phase fixed-bed operations, gaseous reactor effluent comprising unreacted synthesis gas and light hydrocarbon gas can be recycled to improve conversion efficiency and partly to quench the exothermic reaction. One limitation of using light hydrocarbon gases as a quench is their relatively low thermal conductivity.
Supercritical fluids (SCFs) can offer certain advantages over traditional solvents for catalytic reactions including the ability to manipulate the reaction environment through simple changes in pressure to enhance solubility of reactants and products, to eliminate interphase transport limitations, and to integrate reaction and separation unit operations. SCF solvents offer attractive physical properties including; low viscosity and high diffusivity resulting in superior mass transfer characteristics; low surface tension enabling easy penetration into the pores of a solid matrix (catalyst) for extraction of non-volatile materials from within the pores; high compressibility near the critical point inducing large changes in density with very small changes in pressure and/or temperature. These unique properties of SCFs have been exploited to provide many opportunities for the design of heterogeneous catalytic reaction systems.
Elbashir et al, in Proceedings of the 1st Annual Gas Processing Symposium, 2009, pages 1-11 (“An Approach to the Design of Advanced Fischer-Tropsch Reactor for Operation in Near-Critical and Supercritical Phase Media”) describes a reactor system for a super-critical or near-supercritical phase Fischer-Tropsch process. Certain advantages of a supercritical fluid process include gas-like diffusivities and liquid-like solubility which together combine desirable features of the gas and liquid-phase FTS processes. Huang et al in Fuel Chemistry Division Preprints, 2002, 47(1), pages 150-153, report that a supercritical phase reaction can also reduce production of undesirable products; produce less methane because of better distribution of heat in the reactor; produce more long-chain olefins as a result of the enhanced solubility and diffusivity of these higher hydrocarbons in the SCF; mitigate deactivation of the catalyst through better heat and mass transfer; provide in-situ extraction of heavy hydrocarbons from the catalyst surface and their transport out of the pores thereby extending catalyst lifetimes; enhance pore-transport of the reactants such as hydrogen to the catalyst surface thereby promoting desired reaction pathways; enhance desorption of the primary products preventing secondary reactions that adversely affect product selectivity towards longer chain hydrocarbons.
Yan et al in Applied Catalysis A, 171 (1998), pages 247-254, report that a Co/SiO2-catalysed supercritical-phase Fischer-Tropsch process improves extraction of product from the catalyst bed efficiently, and enhances mass transfer for reactants and products. They also report that the addition of 1-tetradecene as a chain initiator can participate in the chain-growth process, which increases the rate of formation of hydrocarbons larger than C14 and decreases the yield of C1-13 hydrocarbons, leading to a flatter carbon number distribution of product compared to that obtained without addition of the olefin.
There remains a need for an improved Fischer Tropsch process improving the yields of hydrocarbons having 10 or more carbon atoms, in particular hydrocarbons having in the range of from 10 to 25 carbon atoms or from 10 to 20 carbon atoms.